Within the field of image recording, such as thermal cameras for example, it is known to make use of an optical line sweep system in which an object is scanned at high speed along a line with the aid of a rotating drum provided with continuous reflecting faces arranged around the drum. An image sweep perpendicular to the line can also be achieved, for example using a rocking mirror or a second drum which rotates at a lower speed than the drum for the line sweep. In this manner the object is scanned in grid form. The radiation is transmitted through such a system via one or more deflection members such as lenses or mirrors to one or more radiation detectors. The radiation detector or detectors also receive radiation from one or more reference bodies which constitute sources of radiation such as, for example, infra-red radiation. It is known to arrange the reference body in such a manner that both the object and the reference body are scanned during one sweep, whereby the image sweep scans the reference body at the start and finish respectively of the sweep.
In most types of optical sweep systems with drums provided with mirrored faces which function as sweep-generating elements, the faces are normally installed so that they reflect in a collimated radiation path. The radiation beam which is reflected by the faces and other deflection members thus has a certain width. There are however sweep systems in which the faces are arranged in convergent or divergent radiation paths.
Even with convergent or divergent radiation paths, the radiation beam must however have a certain width. Because of the width of the radiation beam, difficulties are created with double imaging which arises in connection with the switching of the radiation path between two faces. This arises because radiation during switching from one face to another face will arrive at the detector from two directions, i.e. radiation coming simultaneously from both the rear portion of one face and from the front portion of the next face on the drum. These two radiation beams originate from separate parts of the object and thus give rise to double imaging during a portion of the sweep. This double imaging effect implies that not all of the sweep can be used, which explains why the sweep efficiency of such arrangements is not as high as is desirable.
By analogy with the above, a double imaging effect arises during the transition of the sweep from scanning the object to scanning the reference body, which also implies that the sweep efficiency is not as high as is desirable. In addition, the scanning sweep across the reference body implies that very high demands are placed on uniform temperature distribution over the reference body since it is of utmost importance that the detector always records the same temperature from the reference body.
More recently, new technology having no moving parts for the actual image detection has started to be used, whereby the detector is in the form of a flat detector grouping (Focal Plane Array, FPA) placed in an image plane. Each image pixel is formed by its own detector element. This technology has previously been regarded as expensive, though it is now coming down to a more affordable price. This technology is not completely new and a plurality of FPA-arrangements have been produced which differ primarily in the different detector material. An infra-red camera according to the known technology can incorporate temperature references which are automatically introduced into the radiation path to the detector or detector elements several times per image and from which the infrared camera can calibrate itself.
Those FPA systems which have until now been introduced onto the market do not contain actual temperature references. Neither can they measure temperature particularly well. They do, however, often contain a disc having a uniform temperature which at certain instances can normally automatically be displaced into the radiation beam so that all of the detector elements can be calibrated to emit the same output signal when they observe object points having the same temperature. There are also variations to FPA systems in which the whole camera can be made to observe an external temperature reference disc so that even the amplification from each individual detector element, i.e. each pixel, can be calibrated. These known systems are, however, burdened with the problem that the temperature reference disc must be at least as big as the pupil in the system and placed next to the pupil. Alternatively, it is necessary that the temperature reference disc is maintained at a very uniform temperature so that the various detector elements in the detector grouping are able to read the same temperature. Reading the same temperature is of utmost importance in order to be able to achieve temperature calibration of the image system.